1. Field of the Invention
The present invention generally relates to a semiconductor device and methods of making the semiconductor device and, more particularly, to a thin P-N diode containing an ultra-sharp P-N junction.
2. Description of the Related Art
A conventional crosspoint magnetic tunnel junction magneto-resistive memory cell such as a nonvolatile magnetic random access memory (MRAM), as disclosed for example, in U.S. Pat. No. 5,640,343, commonly assigned with the present application and incorporated herein by reference, requires a diode in series with each magnetic tunnel junction (MTJ) memory element.
With this arrangement, a sense current flows through only one memory element instead of through N elements (where N is an integer greater than 1), as in conventional series architecture magneto-resistive memories. As a result, the signal-to-noise ratio is increased by a factor of N at the same sense power, or alternatively the sense power is decreased by N squared at equal signal-to-noise ratio. Moreover, high density very-large-scale-integrated (VLSI) magneto-resistive memory designs become possible.
However, the areal density of the crosspoint magnetic tunnel junction magneto-resistive memory cells can be increased if the diode is located on top of a metal conductor, as shown in FIGS. 1B, 1C, and 9 of U.S. Pat. No. 5,640,343. The write current, which flows through this lower conductor is on the order of milli-Amperes, and should be close to the magnetic tunnel junction for good efficiency in generating the required magnetic field at the magnetic tunnel junction.
A diode on top of a conductor can be formed by crystalizing a deposited amorphous silicon layer. Such crystalized material has been used for silicon solar cells, but the conductivity of those diodes is lower than required for the crosspoint magnetic tunnel junction magneto-resistive memory cell. Further, since the sensing operation is a resistance measurement, any series resistance or low conductivity switch in series with the sense current path will detract from the signal being sensed. This is a problem.
Additionally, with the conventional structures, it has not been possible to simultaneously provide a high conductivity diode built on top of a metal conductor, and to minimize the contact resistance in series with the diode. Specifically, tradeoffs have been required since to optimize both objects requires thin and highly-doped silicon regions for very steep junctions which conflicts with the conventional processing of crystalized silicon (e.g., "polysilicon").